AIR VESICLES 137 



cartilaginous plates, muscular tissue, fibrous elastic and inelastic 

 tissue and a lining membrane of ciliated epithelium. 



Bronchial tubes -^ in. in diameter, and smaller, have in their 

 walls the same elements excepting the cartilage; but as the tubes 

 subdivide their walls grow continuously thinner, and the inelas- 

 tic tissue become less and less in amount, until it finally prac- 

 tically disappears; the ciliated epithelial cells gradually give 

 place to a single layer of squamous cells in the smallest tubes. 

 The smallest bronchial tubes, the bronchioles, are from T ^j- to 

 -fa in. in diameter. Of course everywhere in the walls there 

 are vessels and nerves. 



The Air Vesicles. Each bronchiole opens into a collection 

 of air vesicles, or cells, called a pulmonary lobule. The term 

 lobulette will be here applied to it, however, reserving. the word 

 lobule for a collection of lobulettes about J in. in diameter. The 

 bronchiole entering the lobulette becomes the infundibulum 

 (Fig. 50), a slightly dilated canal from which are given off from 

 eight to sixteen oblong vesicles, the true air cells. The cells are 

 a little deeper than they are wide and end in blind extremities. 

 The diameter of the lobulette is about -^VrV ^ n> > * na t f the 

 vesicle about 2 oo~7 l o i n - ^ nas been estimated that there are 

 some 725,000,000 of these vesicles in the lungs and that their 

 combined area is something over two hundred square yards. 



The walls of the air cells are very thin, being composed of a 

 single layer of flattened epithelium together with highly elastic 

 fibrous tissue. Ramifying in this latter is a most abundant supply 

 of capillaries, which are larger here than anywhere else in the 

 body. The physical conditions are most favorable for the ex- 

 change of gases between the blood and air, each capillary being 

 exposed to vesicles on both sides, and the air and blood being 

 separated only by the very thin walls of the capillary and vesicle. 

 The elastic tissue is very important in expelling the air from the 

 cells when the inspiratory effort has ceased. 



For the nutrition of the bronchi and lung substance arterial 



